[unreadable] [unreadable] Diffusion is an important transport mechanism in both the extracellular (EC) and intracellular (IC) compartments of brain tissue. The physical properties of these complex environments can be summarized by two parameters: the volume fraction determines the proportion of the brain volume accessible to diffusing molecules and the tortuosity describes the hindrance imposed by the environment relative to an obstacle free medium. Diffusion-weighted magnetic resonance (DW-MR) detects a complicated mixture of the EC and IC signals. The Real-Time Iontophoresis (RTI) method also can be used to measure diffusion but only in the EC space. Both methods have been used to study pathological conditions such as ischemia, and generally found slower diffusion in the compromised tissue. The mechanisms are still debated. [unreadable] [unreadable] We will measure diffusion of a small anion, hexafluoroantimonate (SbF6-), to aid in the interpretation of the DW-MR measurements. SbF6- can be detected by both DW-MR and RTI, and does not easily cross cellular membranes. With help from the RTI method, it will be possible to separate the diffusion properties of the EC and IC compartments. There are three specific aims: [unreadable] [unreadable] Aim 1: Test the hypothesis that the measured diffusion properties of SbF6- are the same in DW-MR and RTI. A fluorine-sensitive DW-MR sequence will be optimized at 7 Tesla to reliably detect the SbF6- anion acting as a water substitute. The diffusion will be measured in a diluted agarose gel using both methods to establish the degree of their correspondence. [unreadable] [unreadable] Aim 2: Test whether differences exist between the EC and IC diffusion parameters in the turtle cerebellum under normal conditions and during hypoosmotic stress mimicking cell volume changes in ischemia. The EC diffusion will be measured by DW-MR and compared with RTI. The DW-MR measurement will then continue for several hours, facilitating gradual entry of extracellular SbF6- into cells. This will allow separation of the EC and IC diffusion properties. [unreadable] [unreadable] Aim 3: Measure EC and IC diffusion in a thick-slice in vitro model of ischemia. The method developed in Aim 2 will be applied to determine the characteristics and origins (EC, IC, or both) of the expected diffusion decrease in the thick (1000 (m) slices of rat neocortex. [unreadable] [unreadable]